Biochar as a sorbent for contaminant management in soil and water: A review

Highlights

• This manuscript reviews recent findings published on biochar.

• Physical architecture and composition of biochars are critical for soil and water remediation.

• Sorption capacity depends on surface area, microporosity, and hydrophobicity of biochar.

• Long-term effects of biochar on contaminant stability in soil should be investigated.

Abstract

Biochar is a stable carbon-rich by-product synthesized through pyrolysis/carbonization of plant- and animal-based biomass. An increasing interest in the beneficial application of biochar has opened up multidisciplinary areas for science and engineering. The potential biochar applications include carbon sequestration, soil fertility improvement, pollution remediation, and agricultural by-product/waste recycling. The key parameters controlling its properties include pyrolysis temperature, residence time, heat transfer rate, and feedstock type. The efficacy of biochar in contaminant management depends on its surface area, pore size distribution and ion-exchange capacity. Physical architecture and molecular composition of biochar could be critical for practical application to soil and water. Relatively high pyrolysis temperatures generally produce biochars that are effective in the sorption of organic contaminants by increasing surface area, microporosity, and hydrophobicity; whereas the biochars obtained at low temperatures are more suitable for removing inorganic/polar organic contaminants by oxygen-containing functional groups, electrostatic attraction, and precipitation. However, due to complexity of soil–water system in nature, the effectiveness of biochars on remediation of various organic/inorganic contaminants is still uncertain. In this review, a succinct overview of current biochar use as a sorbent for contaminant management in soil and water is summarized and discussed.

Introduction

The origin of biochar is connected to the ancient Amerindian populations in the Amazon region, locally known as Terra Preta de Indio, where dark earth was created through the use of slash-and-char techniques (Lehmann, 2009, Lehmann and Joseph, 2009). Research on Terra Preta soils (Hortic Anthrosols) in Amazonia has revealed the effects of biochar on functionalization of soils. Especially, because the biochar has been known as an excellent soil amendment for soil fertility and sustainability, many researchers and farmers worldwide are paying attention to its hidden secrets. Biochar is also recognized as a very significant tool of environmental management (Lehmann and Joseph, 2009).

Biochar is a newly constructed scientific term. According to Lehmann and Joseph (2009), it is defined as “a carbon (C)-rich product when biomass such as wood, manure or leaves is heated in a closed container with little or unavailable air” (Lehmann and Joseph, 2009). Shackley et al. (2012) defined biochar more descriptively as “the porous carbonaceous solid produced by the thermochemical conversion of organic materials in an oxygen depleted atmosphere that has physicochemical properties suitable for safe and long-term storage of carbon in the environment”. Verheijen et al. (2010) also defined biochar as “biomass that has been pyrolyzed in a zero or low oxygen environment applied to soil at a specific site that is expected to sustainably sequester C and concurrently improve soil functions under current and future management, while avoiding short- and long-term detrimental effects to the wider environment as well as human and animal health”. The International Biochar Initiative (IBI) standardized its definition as “a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment” (IBI, 2012). All of these definitions are directly or indirectly related to the biochar production condition and its application to soil. Lehmann and Joseph (2009) distinguished biochar operationally from charcoal. Primarily, the difference between these two terms lies in the end use. The charcoal is a source of charred organic matter for producing fuel and energy whereas the biochar can be applied for carbon sequestration and environmental management. The term hydrochar is closely related to biochar; however, it is distinguished by different condition like the hydrothermal carbonization of biomass (Libra et al., 2011). In general, biochar is produced by dry carbonization or pyrolysis and gasification of biomass, whereas hydrochar is produced as a slurry in water by hydrothermal carbonization of biomass under pressure. The two chars differ widely in chemical and physical properties (Bargmann et al., 2013).

Four major areas where biochar is being used in environmental management include (i) soil improvement, (ii) waste management, (iii) climate change mitigation, and (iv) energy production (Lehmann and Joseph, 2009).

Because of its high organic C content, biochar has the potential to serve as a soil conditioner to improve the physicochemical and biological properties of soils. Soil water retention capacity increases with increase in organic C. About 18% increase in the water holding capacity of soil containing biochar was reported (Glaser et al., 2002). Soil water holding capacity is related to hydrophobicity and surface area of biochar, and the improved soil structure following biochar application (Verheijen et al., 2010). A decrease of nutrient leaching due to biochar application has been also reported (Sohi et al., 2009). Biochar generally has a neutral to alkaline pH; however, acidic biochar has been also reported (Chan et al., 2007). The pH of biochar depends on various factors including feedstock type and the thermochemical process of production. The alkaline pH of biochar induces a liming effect on acidic soils, thereby possibly increasing plant productivity. The extent of liming effect of biochar depends on its acid neutralizing capacity that varies depending on the feedstock and pyrolysis temperature. For example, biochar derived from paper mill waste pyrolyzed at 550 °C had a liming value around 30% that of CaCO₃ (Zweiten et al., 2010). Significant increases in seed germination, plant growth, and crop yields have been reported in the soils amended with biochars (Glaser et al., 2002). Applying biochar together with organic or inorganic fertilizers can even enhance crop yields (Lehmann et al., 2002). Increased microbial population and microbial activity in soils amended with biochar have also been observed (Verheijen et al., 2010, Lehmann et al., 2011). Significant changes in soil microbial communities and enzyme activities influence biogeochemical processes in soils (Lehmann et al., 2011, Awad et al., 2012). The effects of biochar on soil fauna have been scarcely studied besides a number of studies on earthworm activity in soil. Weyers and Spokas (2011) reported that the short-term negative effects altered to the long-term null effects on earthworm population in soils amended with biochar. Negative effects of biochar on earthworm population are postulated to be related to rise in soil pH by biochars derived from sludges, manures or crop residue. However, wood-based biochars showed null to positive impacts on earthworm population (Weyers and Spokas, 2011). Recently, Li et al. (2011) recommended that a wet biochar application to soil could help mitigate avoidance of earthworms by preventing desiccation.

One of the major issues with applying biochar is its priming effect on soil native C (Wardle et al., 2008, Awad et al., 2013). Biochar accelerates the decomposition of soil native C (i.e. the positive priming effect) by improving microbial populations (Kuzyakov et al., 2009) and the chemical hydrolysis by increasing soil pH (Yu et al., 2013). On the contrary, others have shown that the biochar increases the adsorption of dissolved organic C (Kwon and Pignatello, 2005, Zimmerman et al., 2011), thereby decreasing its decomposition rate (i.e. the negative priming effect). This negative priming effect has attributed to the toxicity of biochar, thereby resulting in a decrease in microbial activity (Zimmerman et al., 2011). It presumes that physicochemical properties of biochar such as mobile and resident organic matter, and sorption capacity would influence the priming effect of biochar on soil C. Therefore, it is worthwhile to completely characterize biochar before applying in soil.

Biochar has great potential for managing the waste stream originating from animals or plants; thus, decreasing the associated pollution loading to the environment. The use of waste biomass for biochar production is not only economical but also beneficial. The benefits mainly including energy production and climate change mitigation (Barrow, 2012). Waste biomass that has been extensively used to produce biochar includes crop residues, forestry waste, animal manure, food processing waste, paper mill waste, municipal solid waste, and sewage sludge (Brick, 2010, Chen et al., 2011a, Chen et al., 2011b, Cantrell et al., 2012; Enders et al., 2012). Notably, pyrolyzing the waste biomass, particularly animal manure and sewage sludge, kills any microbes present, thereby reducing the environmental health effects (Lehmann and Joseph, 2009). However, the persistence of toxic heavy metals in biochar developed from sewage sludge and municipal solid waste (Lu et al., 2012) must be carefully handled before long-term application to soils.

Converting biomass into biochar and its application to soils have been proposed as one of the best ways to mitigate climate change by sequestering C in soil (Lehmann et al., 2008). The long-term stability of biochar in soil is a key factor affecting a decrease of CO₂ emissions into the atmosphere (Cheng et al., 2008, Kuzyakov et al., 2009, Singh et al., 2012). A recent long-term experiment estimated that the mean residence time of C in biochars varies from 90 to 1600 years depending on the labile and intermediate stable C components (Singh et al., 2012). A few recent studies have shown that biochar can reduce nitrous oxide (N₂O) and methane (CH₄) emissions from soil by both biotic and abiotic mechanisms (Zweiten et al., 2009). Woolf et al. (2010) proposed a sustainable biochar concept by which emissions of greenhouse gases including CH₄ and N₂O can be avoided by pyrolyzing waste biomass. Additionally, the bioenergy produced during the pyrolysis process offsets fossil energy consumption, and half of the C fixed in biomass during photosynthesis is retained (Woolf et al., 2010). Biochar has been estimated to be capable of offsetting a maximum sustainable technical potential (∼12%) of current anthropogenic CO₂–C equivalent emissions (Woolf et al., 2010).

Another potential use of converting waste biomass to biochar is the production of bioenergy during the fast and slow pyrolysis processes. This bioenergy can be used as an alternative to fossil energy with low fossil CO₂ emissions (Bolan et al., 2013a). However, bioenergy production is dependent on the pyrolysis conditions, in which the slow pyrolysis results in a lower yield of liquid fuel and more biochar, whereas the fast pyrolysis generates more liquid fuel (bio-oil) with relatively less biochar (Mohan et al., 2006). It is assumed that with an intermediate yield of 35% biochar, a maximum bioenergy output of 8.7 MJ kg⁻¹ of biomass could be obtained (Woolf, 2008). However, the production of biochar and/or bioenergy from biomass is still controversial.

Despite the benefits of biochar applications to soil, the mechanisms explaining the interaction between biochar and soil properties have not been fully understood. The long-term effects of biochar applications to different soils should also be monitored (Singh et al., 2012). Both qualitative and quantitative assessments of emissions produced during traditional pyrolysis of waste biomass should be carried out to evaluate their effect on occupational health and safety (Verheijen et al., 2010).

Discharge of environmental contaminants from industrial, residential, and commercial sources degrades the surrounding ecosystems. Soil and water media in an ecosystem are frequently subject to contamination by organic and inorganic contaminants mainly due to anthropogenic activities. Technologies are advancing to remediate contaminated soil and water. One of the most important technologies is to reduce the bioavailability of contaminants, and consequently decrease their accumulation and toxicity in plant and animals. Biochar is emerging as an ameliorant to reduce the bioavailability of contaminants in the environment with additional benefits of soil fertilization and mitigation of climate change (Sohi, 2012).

Environmental remediation has been recognized recently as a promising area where biochar can be successfully applied (Cao et al., 2011, Ahmad et al., 2014). In this review, the effects of pyrolysis conditions, including residence time, feedstock types, temperature and heat transfer rate, on biochar properties, and consequently its efficacy for contaminant remediation are discussed in detail. Special emphasis is given to the mechanistic evidence of the interaction of biochar with soil and water contaminants. Therefore, this review is limited to applying biochar as a green environmental sorbent for the soil and water contaminated with organic/inorganic contaminants.